Electrolyte purification

ABSTRACT

Method of stripping accumulated titanium fines from the electrolyte of an electrolytic titanium cell consisting in adding chlorine gas to the electrolyte to react with the said titanium fines and produce titanium tetrachloride and removing the titanium tetrachloride with the off gases. Said method being carried out either in said cell wherein the chlorine gas is added adjacent to the cell bottom or in an auxiliary vessel. When said method is carried out in the auxiliary vessel, the titanium fines are mixed throughout the electrolyte in said cell and the mixture is pumped into the auxiliary vessel wherein the titanium is chlorinated from the electrolyte.

United States Patent [72] Inventors John C. Priscu;

Eldon R. Poulsen, both of Las Vegas, Nev. [21] App]. No. 793,718 [22] Filed Jan. 24,1969 [45] Patented Sept. 21, 1971 [73] Assignee Titanium Metals Corporation of America West Caldwell, NJ.

[54] ELECTROLYTE PURIFICATION 4 Claims, 3 Drawing Figs.

{52] 11.8. CI. 23/87 R,

23/1D, 23/87 T. 23/89 R, 204/105 R [51] Int. Cl C01g 23/02 [50] Field of Search ..23/87T, 87,

89, l D, 87 TP; 204/105 [56] Relerences Cited UNITED STATES PATENTS 2,858,189 10/1958 Globus, 23/87 2,864,749 12/1958 Slatin 23/87 X 3,067,112 12/1962 Trumpler 204/61 3,399,029 8/1968 Rubel 23/87 X Prima ry Examiner-Edward Stern Attorney-Webb, Burden, Robinson & Webb ABSTRACT: Method of stripping accumulated titanium fines from the electrolyte of an electrolytic titanium cell consisting in adding chlorine gas to the electrolyteto react with the said titanium fines and produce titanium tetrachloride and removing the titanium tetrachloride with the off gases. Said method being carried out either in said cell wherein the chlorine gas is added adjacent to the cell bottom or in an auxiliary vessel. When said method is carried out in the auxiliary vessel, the titanium fines are mixed throughout the electrolyte in said cell and the mixture is pumped into the auxiliary vessel wherein the titanium is chlorinated from the electrolyte.

PATENTED SEPZI I9?! 3,607,011 SHEETQI [1F 2 THE IR ATTORNEYS PATENTEBSEPZI m 7 3,607,011

- sum 2 0F 2 INVENTORS. John C. Priscu Eldon R. Paulsen THEIR ATTORNEYS ELECTROLYTE PURIFICATION Our invention relates to electrolytic titanium cells and more particularly to the removal and recovery of accumulated titanium fines from the cells.

In the operation of electrolytic titanium cells, titanium crystals are deposited on the cathode assembly. When the catl de assembly is removed from the cell at the end of each operating cycle, minute titanium crystals drain from the assembly into the cell electrolyte. A portion of these crystals setties to the bottom of the cell into the sludge sump, and a portion remains in suspension in the electrolyte above the sludge sump. The electrolyte near the bottom of the cell is not in the working area between the anodes and the cathode assembly; and, therefore, the chlorine which is generated at the anodes during normal cell operation does not contact the fine titanium crystals in the sludge sump and in the electrolyte adjacent thereto. The titanium fines in the sludge sump and in suspension immediately above the sludge sump gradually increase in density, and eventually the path of least resistance to the electric current is through the accumulated fines. This causes partial short circuiting of the cell and a decrease in cell efficiency. Also, a less desirable crystal structure is obtained in the titanium deposited on the cathode assembly when partial short circuiting occurs.

The past practice employed to remove the titanium fines was to periodically desludge the cell witha mechanical device such as a clamshell bucket or a cold probe. While these methods removed sufficient titanium fines from the cell to avoid partial short circuiting, they were time consuming and dangerous. Additionally, a large portion of the titanium fines was lost through oxidation when removed in this manner. Also, the graphite anodes extending upwardly from the bottom of the cell were often damaged by contact with the mechanical sludge removing equipment.

Our invention provides novel methods for stripping titanium fines from an electrolytic cell and has a number of advantages over mechanical desludging. One embodiment of the method consists in the introduction of chlorine gas directly to the lower portion of the cell. The chlorine concentration should be as high as possible to avoid cell damage and to achieve maximum efficiency. A concentration of 98 percent has been used successfully although this concentration is not critical. The chlorine reacts with the titanium fines to convert the titanium to TiCl,. Continued introduction of chlorine converts the TiCl, to TiCl and then to TiCl, which passes from the cell at the approximate rate of chlorine introduction. Chlorineintroduction is continued until the electrolyte is substantially free of titanium at which time the cell is again ready for normal operation. The TiCl and any chlorine in the off gas are separated in a refrigerated scrubber condenser, and the TiCl, is resupplied to the cell. The chlorine gas is recovered and may be used as desired.

In an alternative embodiment of our method, an agitator is placed in the cell sludge sump to mix the titanium fines throughout the electrolyte. The mixture of electrolyte and fines is then pumped from the cell into an auxiliary vessel which includes electrodes, and the titanium is then converted to TiCl and passes out of the vessel. This embodiment is ad vantageous in pot line operation as it does not tie up the electr'olytic cell during desludging, and the cell may be provided with clean electrolyte and operated in the normal manner.

The advantages of our method include the elimination of the inherent danger involved in using mechanical removal equipment as well as the elimination of harmful fume. Additionally, our method does not require exposure of the cell members to the atmosphere, and there is no possibility of damaging the anodes by contact with mechanical removal apparatus. The method results in substantially complete recovery of all titanium fines from the cell as TiCh, and there is no loss of electrolyte.

In the accompanying drawings, we have shown apparatus which may be used in practicing preferred embodiments of our invention. In these drawings:

FIG. 1 is a vertical section through an electrolytic cell with the gas sparger assembly in position;

FIG. 2 is a section on line llll of FIG. I with the fines omitted from the sump; and

FIG. 3 is a vertical section through an electrolytic cell and an auxiliary vessel which is used in an embodiment of our invention.

FIG. 1 if the drawings shows an electrolytic cell having a steel shell 1, a refractory lining 2, a sludge sump 3, a plurality of anodes 4 and containing a fused halide salt electrolyte 5. in normal operation, the cell includes a cathode assembly such as described in US. Pat. No. 3,282,822, but in practicing our invention, the cathode assembly is removed. In place of the cathode assembly, the cell in FIG. 1 is provided with a chlorine sparger assembly 6. The sparger assembly has an upper section 7 connected with a lower nozzle section 8 by coupling 9. Upper'section 7 extends through a cover 10 and is supported on the cover by a threaded flange or other convenient means which permit rotation of the assembly relative to the cell walls. The lower end of nozzle section 8 is provided with four outlet passages 11 for supplying chlorine gas adjacent the bottom of sump 3. The outlets are preferably spaced from each otherso that the chlorine contacts the titanium fines in all parts of the cell as the assembly is rotated. The lower end of the nozzle section preferably rests on the cell bottom wall to prevent the sparger assembly from whipping during chlorine introduction.

As shown in FIG.- 1, anodes 4 terminate slightly below the surface of the electrolyte. The lower ends of two groups of anodes are connected to a source of alternating current (not shown) to keep the cell hot. The top of the cell is covered by a metal plate 12 which has a channel 13 filled with a liquid sealing metal 14 which may be a lead or tin alloy with a low melting point. A downwardly extending flange 15 on cover 10 extends into the metal in channel 13 when the cover is in place.

The sealing arrangement is described and claimed in US. Pat. No. 2,871,178, and the details of this arrangement form no part of the instant invention.

After the cathode is removed from the cell at the end of a normal electrolysis cycle and it has been determined that titanium fines have accumulated in the sludge sump and the electrolyte in an amount sufficient to adversely affect the operation of the cell, chlorine sparger assembly 6 is inserted into the cell, and chlorine gas is supplied to remove the titanium fines from the sludge sump and the electrolyte. The sparger assembly is rotated about l5 approximately every 4 hours to insure circulation of chlorine to all portions of the sump and the electrolyte. Optimum removal of the titanium fines takes place with this operation.

When sparger assembly 6 is in position in the cell, concentrated chlorine gas is supplied through outlet passages 11 at a rate which insures effective reaction of the chlorine gas with the titanium fines. Sufficient current is supplied to anodes 4 to maintain the electrolyte about 25 to 50 C. above its melting point. As explained above, chlorine gas reacts with the titanium fines to convert the titanium to titanium dichloride then to titanium trichloride. As a result of continued introduction of chlorine to the cell, the titanium trichloride reacts to titanium tetrachloride. The titanium tetrachloride passes from the cell through outlet pipe I7 at the approximate rate of chlorine introduction. The off gas from the cell is passed to a refrigerated scrubber condenser (not shown) where any chlorine present with the titanium tetrachloride is separated therefrom. The introduction of chlorine gas into the cell is continued until such time as analysis of the off gas indicates that the electrolyte has been substantially completely stripped of titanium which is shown by an excess of more than about 10 percent chlorine in the off gas. This is easily determined by monitoring the off gas in a manner well known to those skilled in the art. The length of time required to strip the electrolyte will vary with the amount of fines in the electrolyte so that more frequency chlorination will result in shorter chlorinating periods.

This embodiment of our method may be best understood by reference tothe following nonlimiting example.

A clean electrolytic cell was operated to produce titanium metal until the efficiency of the cell decreased appreciably. At

this time, samples were taken from the sludge sump and the electrolyte, and a large accumulation of titanium fines was found to exist. It was determined from measurements of titanium finesraccumulated at this stage in the cell operation that about 200 pounds of titanium fines would have to be removed to strip the electrolyte. A graphite sparger assembly having four outlet passages arranged to inject chlorine into the electrolyte was installed through the top of the cell. The sparger assemblywas placed in the cell with the lower end resting on the cell bottom to prevent whip due to the surging action of the gas during chlorine introduction and possible breakage of the graphite nozzle section. The temperature of the cell was maintained between 825 and 875 C. by applying alternating current across the cell from the anodes on one side of the cell to the anodes on the other side. The rate of chlorine introduction was controlled through a standard flow meter (not shown) located in the chlorine supply pipe to prevent chlorine gas or titanium chlorides from escaping from the cell until substantially all the titanium fines were converted to TiCl, and TiCl,. The off gas from the cell was continuously monitored for both chlorine and titanium tetrachloride. After approximately400 pounds of chlorine had been supplied to the cell over a period of about 60 hours, TiCl appeared in the off gases. The chlorine'flow through the cell was continued until 757 pounds of TiCl, had been recovered at which point excessive free chlorine was showing in the cell off gases, and the chlorine flow was stopped. About 120 hours were required to remove the 757 pounds of TiCl, with a chlorine feed rate of pounds per hour for the first 100 hours and 40 pounds per hour for the last hours.

After 120 hours of operation, the chlorine flow was stopped, the chlorine feed pipe removed and the cell emptied of electrolyte. Inspection of the cell showed it to be free of titanium deposits with the exception of some very small deposits in the lower corners between the outer periphery of the anodes and the cell wall. The electrolyte removed from the cell was analyzed" and found to be completely free of free titanium although traces of titanium dichloride appeared in the electrolyte which is normal.

This embodiment of our invention may also be carried out in a manner to reduce the total time for stripping the electrolyte to a period of about 24 to 36 hours which is advantageousas it reduces the downtime of the cell. This modification is based upon the principle that the chlorine gas has not solubility in the sodium chloride electrolyte, and, therefore, it must react to form either TiClg or TiCl in order to be absorbed into the electrolyte. In order to react with titanium, the chlorine must come into physical contact with it, and chlorine reacts more readily with TiCl which is dissolved in the electrolyte and, therefore, more widely and evenly dispersed than the .titanium to convert the TiCl to TiCl which diffused throughout the electrolyte thus contacting titanium deposits which could not be reached with chlorine supplied to the elec trolyte from the sparger assembly. After all the titanium has been thus converted to TiCl continued addition of chlorine converts the TiCl to TiCl which will not remain dissolved in the electrolyte and readily flows out of the cell as a gas. The modification to this embodiment of our invention requires termination of the electrolysis run immediately prior to the post electrolysis period which leaves TiCl -TiCl mixture in the electrolyte. Normally, at the end of every electrolysis run, current is supplied after TiCl feed is stopped. This is known as the post electrolysis portion of the run and is carried out to reduce all of the TiCl TiCl mixture to titanium metal. If the post electrolysis portion of the run is eliminated, TiCl -TiCl mixture will remain in the electrolyte when the cathode is removed. Bylleaving TiCl TiCL, mixture in the electrolyte, it is possible to start the chlorination at a rate of chlorine introduction between 40 to 50 pounds per hour and thereby cut the overall chlorination time to about 24 to 36 hours. By utilizing this modification, the preliminary period of low chlorine feed is eliminated which results in an appreciable saving in time and chlorine gas requirements.

Another embodiment of our method which may be successfully utilized to strip the electrolyte is carried out in the apparatus shown in FIG. 3 of the drawings where like reference numerals designate the same parts as in FIGS. 1 and 2. When the cell has an accumulation of titanium fines in the electrolyte and in sump 3, an agitator 20 having a blade 21 driven by a motor 22 supported on cover 10 is positioned in the sump. Additionally, a pipe 24 extends downwardly through the cover into the cell. The upper end of pipe 24 is attached to a conventional pump 25 supported on cover 10, and the outlet of the pump is connected with a pipe 26 which extends to an auxiliary vessel 30. The auxiliary vessel is provided with electrodes 32 and 33 connected across a conventional alternating current source (not shown).

In carrying out the method of this embodiment, agitator blade 21 is rotated by motor 22 to disperse the titanium fines throughout the electrolyte. After the fines are thoroughly mixed in the electrolyte, the electrolyte is pumped from the cell into auxiliary vessel 30 through pipes 24 and 26. When substantially all of the electrolyte from cell 1 has been pumped into the auxiliary vessel, the pipe 26 is removed, and a sparger assembly similar to assembly 6 in FIGS. 1 and 2 is inserted through the cover of vessel 30, and chlorine is supplied to the electrolyte while current is supplied to electrodes 32 and 33 to heat the electrolyte. The titanium is chlorinated from the electrolyte in vessel 30 as TiCl and the TiCl passes from the vessel through an outlet pipe (not shown). The TiCl is reused, and the clean electrolyte is returned to an electrolytic cell.

This alternative technique is advantageous in potline operation since the contaminated electrolyte is stripped of titanium in the auxiliary vessel, and the cell can be filled with new electrolyte and operated to produce titanium metal while the electrolyte is stripped. This minimizes cell downtime permitting maximum production per cell day since it takes only about 2 hours to pump the contaminated electrolyte from a cell into an auxiliary vessel as compared with a minimum of about 24 hours to strip the electrolyte of fines. Additionally, the auxiliary vessel is considerably less expensive to construct and to operate than the titaniumproducing cell, and it is, therefore, desirable to strip the electrolyte in an auxiliary vessel.

Our method has numerous advantages in that it effects a substantial savings in time and money and eliminates many of the hazards involved to both personnel and equipment in mechanically removing titanium fines from a cell. The equipment required to practice the invention is inexpensive consist ing only of the sparger assembly and, in the alternative embodiment, a simple agitator and a pump. The chlorine gas is completely recovered, and, hence, there is no expense accrued in this regard.

While we have described and shown preferred embodiments of our invention, it should be understood that the invention may be otherwise embodied within the scope of the appended claims.

1. A method of purifying the fused halide salt electrolyte of an electrolytic titanium deposition cell by chemically stripping accumulated titanium fines from said electrolyte, said method comprising:

1. maintaining said electrolyte at a temperature above its melting temperature so that said electrolyte is molten;

2. introducing concentrated chlorine gas into said molten electrolyte to sequentially react with a portion of said accumulated titanium fines to first form a mixture of TiCl and TiCl which mixture reacts with additional titanium fines to form additional TiCl and all the TiCl subsequently reacts with continued chlorination to form TiCl and whereby the said TiCl and TiCl being produced are soluble in said molten electrolyte and diffuse throughout said electrolyte and the said TiCl, being produced is insoluble in said electrolyte and readily flows out of the cell as a gas;

3. continuously removing said insoluble TiCl from said molten electrolyte; and

4. continuing the introduction of chlorine into said molten electrolyte until essentially all of said titanium fines have reacted to form TiCl and passed from said electrolyte.

2. A method as set forth in claim 1 wherein said molten electrolyte is contained in the electrolytic titanium deposition cell and introducing said chlorine gas adjacent to the bottom of said cell.

3. A method as set forth in claim 1 wherein said chlorine gas is introduced into said electrolyte at a rate between about pounds per hour and about 40 pounds per hour.

4. A method of purifying the fused halide salt electrolyte of an electrolytic titanium deposition cell by chemically stripping accumulated titanium fines from said electrolyte, said method consisting of:

l. stirring said electrolyte to disperse said titanium fines throughout said electrolyte;

2, pumping said mixture of electrolyte and titanium fines from the cell into an auxiliary vessel;

-3. maintaining said mixture at a temperature above the melting point of said electrolyte so that said electrolyte is molten;

4. introducing concentrated chlorine gas into said molten electrolyte to sequentially react with a portion of said accumulated titanium fines to first form a mixture of TiCl; and TiCl which mixture reacts with additional titanium fines to form additional TiCls, and all the TiCl, subsequently reacts with continued chlorination to form C1,, and whereby the said TiCl, and TiCl, being produced are soluble in said electrolyte and diffuse throughout said electrolyte and the said TiCl being produced is insoluble in said electrolyte and readily flows out of the cell as a gas;

5. continuously removing said insoluble TiCl from said molten electrolyte; and

6. continuing the introduction of chlorine into said mixture until essentially all of said titanium fines have reacted to form TiCl and passed from said electrolyte. 

2. introducing concentrated chlorine gas into said molten electrolyte to sequentially react with a portion of said accumulated titanium fines to first form a mixture of TiCl2 and TiCl3, which mixture reacts with additional titanium fines to form additional TiCl3, and all the TiCl3 subsequently reacts with continued chlorination to form TiCl4, and whereby the said TiCl2 and TiCl3 being produced are soluble in said molten electrolyte and diffuse throughout said electrolyte and the said TiCl4 being produced is insoluble in said electrolyte and readily flows out of the cell as a gas;
 2. pumping said mixture of electrolyte and titanium fines from the cell into an auxiliary vessel;
 2. A method as set forth in claim 1 wherein said molten electrolyte is contained in the electrolytic titanium deposition cell and introducing said chlorine gas adjacent to the bottom of said cell.
 3. maintaining said mixture at a temperature above the melting point of said electrolyte so that said electrolyte is molten;
 3. A method as set forth in claim 1 wherein said chlorine gas is introduced into said electrolyte at a rate between about 10 pounds per hour and about 40 pounds per hour.
 3. continuously removing said insoluble TiCl4 from said molten electrolyte; and
 4. introducing concentrated chlorine gas into said molten electrolyte to sequentially react with a portion of said accumulated titanium fines to first form a mixture of TiCl2 and TiCl3, which mixture reacts with additional titanium fines to form additional TiCl3, and all the TiCl3 subsequently reacts with continued chlorination to form TiCl4, and whereby the said TiCl2 and TiCl3 being produced are soluble in said electrolyte and diffuse throughout said electrolyte and the said TiCl4 being produced is insoluble in said electrolyte and readily flows out of the cell as a gas;
 4. continuing the introduction of chlorine into said molten electrolyte until essentially all of said titanium fines have reacted to form TiCl4 and passed from said electrolyte.
 4. A method of purifying the fused halide salt electrolyte of an electrolytic titanium deposition cell by chemically stripping accumulated titanium fines from said electrolyte, sAid method consisting of:
 5. continuously removing said insoluble TiCl4 from said molten electrolyte; and
 6. continuing the introduction of chlorine into said mixture until essentially all of said titanium fines have reacted to form TiCl4 and passed from said electrolyte. 